1. Field of the Invention
The present invention relates to a discharge lamp lighting circuit, and more particularly to a discharge lamp lighting circuit operable to control the current limitation of a switching element provided on the primary side of a transformer in the discharge lamp lighting circuit comprising a DC—DC converting circuit having a flyback type structure.
2. Description of the Related Art
There is known a structure in which a DC—DC converting circuit, a DC-AC converting circuit and a starting circuit are provided in the lighting circuit of a discharge lamp, such as a metal halide lamp. For example, a PWM (pulse width modulation) method and a PFM (pulse frequency modulation) method have been known as a control method of a switching power circuit constituting a DC—DC converting circuit (a DC—DC converter)
In the PWM method, an ON/OFF ratio or a duty ratio (or a duty cycle) for a switching element constituting a DC—DC converting circuit can be variably controlled to change an output voltage. In the PFM method, moreover, a frequency related to the ON/OFF control of the switching element can be variably controlled to change an output voltage.
A structure of a flyback type is used for the DC—DC converting circuit, for example, in which a semiconductor switching element is connected to the primary winding side of a transformer for conversion to carry out the ON/OFF control of the same element and a rectifier diode and a smoothing capacitor are provided on the secondary side of the transformer.
In the case in which a structure of a voltage resonance type is employed in the flyback type circuit (converter), an inductor is connected as an inductive element to the primary winding of a transformer in series and a capacitor is connected as a capacitive element to a switching element in parallel, for example. Consequently, the breakdown voltage of the switching elements causes a problem.
In the structure of a voltage resonance type, although it is possible to produce an advantage that a loss in the switching element can be decreased, there is a drawback that the breakdown voltage of the switching element is required. For example, in the use of an FET, a drain—source voltage is increased more than that in the case in which the inductive element and the capacitive element are not provided.
By taking, as an example, the lighting circuit of a small-sized discharge lamp (a rated power of 35W) to be used for the light source of a lighting unit for a car, the output voltage of a circuit to be applied before turning ON the discharge lamp is generally set to be approximately 350 to 400 V. Accordingly, it is necessary to select a switching element having a very high breakdown voltage.
For this problem, for example, there can be proposed a method of increasing the winding ratio of a transformer. However, this produces a result contrary to an original purport that a power loss is reduced in the case in which an electrical efficiency is deteriorated by an increase in the value of a current (a primary current value) flowing to the primary winding of the transformer and the switching element. Moreover, when the primary current value is increased, a resonance energy is increased so that a resonance voltage is raised.
It is an object of the invention to provide a discharge lamp lighting circuit comprising a DC—DC converting circuit having a flyback type structure in which the breakdown voltage of a switching element provided on the primary side of a transformer can be reduced and the electrical efficiency of the circuit can be prevented from being deteriorated by the reduction.